Hi,
There are some really good threads on this subject over in the exercise section of the forum. I only understand the basics (they go into it in much more detail, if you want to settle down with a cuppa and read up I'm them).
However, this is my understanding of the basics - but I hope someone else will pop along and correct me if I get anything wrong! Thanks in advance, whoever you are...
Basically, regular exercise will have both long term and shorter term benefits on you BG levels.
The long term benefits are reduced insulin resistance, increased muscle mass, lower fasting BG levels.
The shorter term benefits are that it raises your metabolic rate for the next 24 hrs (thus lowering your BG levels).
Both types of exercise are good, however, there is a difference between how your body reacts to steady exercise (your swimming) and harder exercise (cycling up hills).
The gentle exercise uses up glucose already in your blood stream, with a proportionate drop in BG.
The harder exercise raises your heart rate more, uses the glucose in your blood up faster, and your body thinks 'ooh, this is hard work, and we're running out of glucose to fuel the muscles. I know, I'll release stored glucose from the liver, to help with this problem...' This creates the rise in BG you observed.
The good news is that your muscles are now all fired up, your metabolic rate has been raised, and your BG will fall again quite quickly, and you will still get nicely lowered glucose levels later in the day.
But don't take my word for it! Map it out with your monitor.
I'm coming round to the idea of an evening trundle, every evening, if I've eaten any carbs. A sort of pre-emptive BG suppression, even before I hit the 2 hour BGtest. It'll be interesting to see what effect it has...